1. Field of the Invention
The present invention relates to wiring structures and, more particularly, to techniques for the fabrication of copper (Cu)-filled wiring structures and to the improvement of their performance.
2. Description of Related Art
The deposition of copper (Cu) into the narrow copper-filled lines and vias in current high density wiring technologies is accomplished by two quite different key copper deposition steps. The first of these steps, known as seed copper deposition step, involves the sputter deposition (PVD) of a thin layer of copper. The copper thus deposited is highly pure, but the deposition is not highly conformal and consequently, it cannot be used to deposit all of the copper required by the structure to be formed because voids would result, especially in the vias. To compensate for this deficiency, after the deposition of a thin seed layer, the remainder of the copper required is deposited using a second, very different electroplating step. While the copper formed by electroplating is substantially less pure, the electroplating process itself is sufficiently conformal to fill the structure without voiding.
It should be noted that electroplating cannot be used alone, without the prior deposition of the seed copper layer because a seed copper layer is required to carry the electric current necessary to initiate the electroplating process.
A high density wiring structure comprises lines and vias. As the filling of the vias in the structure is more difficult than the filling of lines, in the discussion below we shall address the problem of successfully filling vias with void-free Cu. However, it should be understood that lines are also formed and filled by the same copper deposition process or processes that are employed to fill the vias in the structure. For example, in general, conventional techniques used for fabricating a Cu-filled via typically involve first forming a via hole in a dielectric matrix in which the wiring structure is embedded. Second, the via hole is lined with a diffusion barrier to prevent the diffusion of Cu into the dielectric. This diffusion barrier typically includes tantalum nitride (TaN) deposited directly onto the dielectric, and tantalum (Ta) deposited on top of the TaN. Third, a thin layer of seed Cu is sputter deposited onto the exposed Ta surface, in order to prepare the via for electroplating. Fourth, an electroplating process is used to fill the via with Cu.
The fabrication of the wiring structure thus involves two quite dissimilar processes, the PVD process, performed in a high vacuum cluster tool, and the electroplating, an aqueous solution process. It would be desirable from a number of standpoints if the electroplating step could be eliminated and the PVD process could be used to deposit all of the requisite copper. This approach would first reduce the cost of manufacturing by obviating the need for electroplating tooling. Second, Cu electroplating baths are extremely toxic. If electroplating could be avoided, the expense and risks associated with handling such materials could be eliminated. Third, as noted above, the copper deposited by electroplating is less pure than that deposited by PVD.
Typical impurity levels in the electroplated Cu used to form wiring structures are: carbon (C), 100 parts per million (ppm), chlorine (Cl), 80 ppm, oxygen (O) 80 ppm, and sulfur (S), 50 ppm. These impurities all degrade the conductivity of the copper, and chlorine can in addition cause corrosion which can subsequently lead to the failure of the structure. By contrast, PVD deposited copper typically has impurity levels of less than 20 ppm for each of the aforementioned elements.
In contrast to the impurities C, O, Cl, and S listed above, which are deleterious to the performance of the wiring structure, some impurities such as boron (B) and phosphorus (P) can be beneficial. In particular B and P are useful in suppressing electromigration. If all of the copper in the wiring structure is to be deposited by PVD, and it is deemed desirable to dope the copper with beneficial impurities, it is straightforward to do so with precise control over the concentration of the beneficial impurities (dopants) by the simple method of incorporating them into the Cu sputter target in the requisite concentration. The processing steps described herein below are independent of whether or not the Cu target used for the PVD process contains deliberately introduced dopants. To simplify the discussion below we will take the term “pure copper” to refer to copper which is free of deleterious impurities, such as C, O, Cl or S, with the understanding that “pure copper” may contain deliberately introduced dopants.
In view of the above, it is desirable to develop a technique which provides high purity Cu filled wiring structures using solely a high purity PVD deposited copper process.